Patent GB-661867-A (GB)
Title: Termination for coupling two-wire to four-wire telegraph lines
IPC: H04L5/16
Filed: 1947-12-31
Abstract: 661,867. Printing telegraphy. STAATSBEDRIJF DER POSTERIJEN TELEGRAFIE EN TELEFONIE. Dec. 31, 1947 [Aug. 3, 1946], No. 34976/47. [A Specification was laid open to inspection under Sect. 91 of the Acts, Dec. 31, 1947]. Class 40 (iii). A telegraph coupling arrangement for coupling a two-wire local circuit to a four-wire trunk circuit comprises a resistance in the local line to the opposite ends of which are connected the control grid and cathode of a thermionic valve, while a further electrode (e.g. anode or screen grid) is coupled to a voltage source controlled by the received signal and so arranged that whether a mark or space is received on the trunk circuit the 'conductivity of the valve remains unvaried. In the idle (spacing) condition tone is received at trunk terminals 5, 6, Fig. 1, and after rectification supplies screen-grid potential to a valve B<SP>1</SP>. This valve has its control grid bias derived from a resistor R<SP>1</SP> in the local line 1, 2 so that the normal rectified current in the line biasses the valve to cut-off. The consequent rise in anode potential at a point X controls a modulator M so as to allow tone from terminals 7, 8 to reach the outgoing trunk 3, 4. When the local station transmits, teleprinter contacts ZC open for marks and remove the cut-off bias from the valve B<SP>1</SP>, causing it to conduct and thus lowering the potential of the point X so that the modulator M prevents tone going to line 3, 4. When the distant station is transmitting, interruptions of tone at terminals 5, 6 remove the rectified current in the local line and the valve B<SP>1</SP> has its negative control grid bias removed. Since the screen grid potential is simultaneously removed however, the valve remains non-conducting and the normal tone is still transmitted from the outgoing trunk terminals 3, 4. In practice the capacity of the local line 1, 2 prevents the removal of grid bias and screen potential being simultaneous. This difficulty might be removed by supplying the screen grid from a separate rectifier dissociated from the local line but the difficulty remains that the power furnished by the tone received at terminals 5, 6 has to be rather high. The circuit of Fig. 2 is designed to overcome these difficulties. In the idle condition tone received at terminals 5, 6 supplies screen grid potential to a valve B<SP>2</SP> which is in series with the local line 1, 2 and obtains its anode potential from a series telegraph battery via the normally closed printer contacts ZC. The valve therefore conducts and the potential drop across resistor R<SP>1</SP> applies - 40 v. to the cathode of valve B<SP>1</SP>. Since the control grid potential of this valve is - 50 v., it does not conduct and tone from terminals 7, 8 is passed to only the left-hand primary winding of the outgoing trunk transformer TR<SP>2</SP>, and tone is transmitted. When a mark (not tone) is received at trunk terminals 5, 6 the screen potential of B<SP>2</SP> is removed and the valve ceases to conduct, recording a mark on the receiving magnet OB. The absence of rectified current at 50 removes both screen and anode potential from the valve B<SP>1</SP> which remains non-conducting to transmit a tone at terminals 3, 4. When the local printer contacts ZC open to transmit a mark, the valve B<SP>2</SP> is cut-off and the potential of the cathode of B<SP>1</SP> falls to - 60 v. since it is connected to resistor R<SP>1</SP> and no current flows in the latter. Valve B<SP>1</SP> now conducts, anode and screen potential being derived from the tone normally received at terminals 5, 6, and tone from terminals 7, 8 is passed to both primaries of output transformer TR<SP>2</SP> in opposition. No tone is therefore sent to line 3, 4, which is the required condition. Since the valve B<SP>2</SP> is a pentode its operation is substantially independent of the anode load, i.e. of the length of the local line to which the termination is connected. The valve B<SP>1</SP> changes from a conducting to a non-conducting state for a small grid swing and restores the signals it sends on to a substantially rectangular shape. The anode current of the valve B<SP>2</SP> is stabilized by deriving a fixed grid bias from a potentiometer R6, R6 across the telegraph battery. Any increase in anode current increases the voltage drop across the resistor R<SP>1</SP> and so decreases the negative cathode potential (say from - 40 v. to - 35 v.) and so tends to reduce the anode current to its former value.